A 45-year-old man walking into a hotel lobby for the first time is suddenly gripped by absolute certainty that he has stood in that exact spot before, right down to the pattern of the carpet, even though he has never visited before and knows this is impossible. The feeling passes within seconds and he is only mildly unsettled by it. Before revealing the answer: what is this, and what specifically about his account makes it not a delusion?
This is déjà vu. What rules out a delusion is that the conviction is brief, and — critically — he knows throughout that it cannot actually be true; the certainty is a passing feeling, not a fixed false belief he continues to hold and reason from afterward, which is exactly what would distinguish it from a delusion of any kind.
References
- Casey, P. & Kelly, B. (2019). Fish's Clinical Psychopathology, 4th ed. Ch. 5, 'Disorders of Memory'.